Significance of the proposed research. Colorectal cancer is a common form of cancer in the United States and is responsible for nearly a million deaths each year worldwide. Improved chemotherapeutics and chemopreventive agents are urgently needed. The nuclear factor kappa-light-chain-enhancer of activated B cells (NF-?B) transcription factor complex is a key promoter of cell survival and tumorigenesis. NF-?B activation is strongly correlated with the development of colorectal cancer. NF-?B inhibitors are desired and are likely to prevent cellular progression to colon cancer. We have discovered a bacterial protein that selectively alters the transcriptional activity of NF-?B in intestinal cells. This protein, NleH1, expressed by Escherichia coli O157:H7, binds to and inhibits the function of a newly identified subunit of NF-?B, the ribosomal protein S3 (RPS3). RPS3 normally guides the recruitment of the NF-?B complex to specific gene promoters. NleH1 functions by reducing the nuclear abundance of RPS3 to dampen subsequent host transcriptional outputs. RPS3 also plays an important role in DNA repair and regulates tumor metastases. Importantly, RPS3 is also upregulated in colorectal cancers. Our proposed studies of a bacterial protein with a natural intestinal cell tropism that binds and inhibits the activities of both RPS3 and NF-?B are likely to have significant utility in the future development of colorectal chemotherapeutics Our long-term goal is to characterize the NleH1 protein and use the information we obtain to design novel therapies for colorectal cancers, in which NF-?B and RPS3 are virtually universally misregulated. The objective of this particular application is to test the hypothesis that delivering NleH1 to colon cancer cell lines will inhibit cellular proliferation. The specific aims are to: 1) quantify the extent to which NleH1 reduces the nuclear abundance of RPS3 in colon cancer cell lines and 2) quantify the extent to which NleH1 inhibits the transcriptional activities of RPS3 and reduces cellular proliferation of colon cancer cell lines. The proposed research is significant and innovative because it will evaluate the utility of a translocated bacterial effector protein in inhibiting host transcriptional activities associated with cancer development. These studies will have a significant positive impact on colon cancer therapeutics, as they will form the basis both for future mechanistic studies in which the specific targeting of NleH1 to colon cancer cells is optimized and also for functional dissection of the region of NleH1 responsible for inhibiting RPS3 and NF-?B.